This invention relates to a dielectric band-stop filter comprising two or more transmission line resonators of the coaxial type.
It is known that a ceramic resonator comprises a basic structure, where a hole is made in a ceramic block of a material with a high dielectric constant, e.g. titanate, the block having side surfaces, a top surface and a bottom surface, and the hole extending from the top surface to the bottom surface. The surfaces of the block are coated, except for the top surface, with an electrically conducting material. Circuit patterns are applied to the top surface, the circuits capacitively coupling a signal to the resonator and outputting the signal. The structure forms a transmission line resonator whose resonance frequency is determined by the length of the hole, i.e. by the thickness of the block. Usually the length of the hole is dimensioned so that a transmission line resonator of a quarter-wave length is obtained. When several holes are made in the block it is possible to realize a band-pass filter with several nodes, but the number of zeroes is limited to one, because it is difficult to isolate from the other resonators a resonator corresponding to a zero. Thus band-stop filters realized in ceramic technology became commercially available only recently. It is characteristic to all these known band-stop filters, that the filter is composed of separate resonators or of the basic structures described in the introduction, where a ceramic block contains a hole and where the block at least on the side and bottom surfaces is coated with conducting material. A desired number of these coated separate reasonator blocks are arranged in a row, whereby a band-stop filter is obtained with a desired bandwidth and center frequency. In a sense each resonator block forms a draining circuit, and these draining circuits are then coupled in a series through inductive or reactive circuits, connecting the upper ends of the resonators using e.g. a separate transmission line length. It is necessary to use separate resonators coated on the outside, because otherwise the mutual inductive and capacitive leaks between the resonators are difficult to control, i.e. in order to obtain sufficient isolation between the resonators. The isolation between the separate resonators is formed with the same coating, which forms an effective partition between the blocks.
A disadvantage of a band-stop filter assembled of separate resonators is that a filter made of many blocks requires a high production capacity, because every block is separately sintered and coated, and the blocks are electrically individually connected to each other, usually by soldering the connecting wires by hand. Further the separate blocks must be fastened to some mounting support in a mechanically reliable way.
In principle it would be possible to make a band-stop filter comprising several resonators in a single ceramic block. Then the distance between the resonator holes must be made very large, resulting in a very bulky filter. This would increase material costs, and a big size is also otherwise inconvenient in portable radio equipment.
U.S. Pat. No. 4,823,098, Motorola, describes a monolithic ceramic filter with band-stop characteristics. The filter comprises 7 resonators located in the same ceramic block, of which three operate as a band-stop filter and the other as a band-pass filter. The resonators in the band-stop section are interconnected via quarter-wave transmission lines. The transmission lines invert the impedance of the resonators, so that the resonators generate zeroes in the filter. It is stated in the publication, that by sawing it is possible to separate said three filters from the block and to coat the new side wall obtained in the cutting with conducting material, whereby the obtained filter operates as an independent band-stop filter with several zeroes. It is not mentioned in the publication what influence the inductive coupling between the resonators, effected through the ceramics, has on the filter characteristic, but it seems probable that mutual coupling between the resonators makes it difficult to control the characteristics.
The Finnish patent applications FI-892855 and FI-892856, applicant LK-Products Oy, describe band-pass filters realized in a single ceramic block, where the basis of the inventive idea is that one side surface of the filter is substantially uncoated and that strip conductor patterns are applied on this side surface for connections to the transmission line resonators. When the circuit patterns are made on the side surface of the body, the filter input and output and the connections between the resonators can be made in a desired way, either purely capacitive or inductive, or as a combination of these.